The present invention relates generally to printing devices and methods and, more particularly, to developing units used in printing devices, e.g., printers.
Printing devices play many roles in today's technology society. Local printers, for example, are coupled directly to (or via a network of some type) most personal computers to provide hard copy output capabilities. Larger scale printers, e.g., digital printing presses, are used commercially to print everything from brochures, mass mailings to newspapers, etc.
One type of printing technology is multicolor electrostatic printing. This printing technology has been used in various commercial products, such as digital printing presses. Parts of an electrostatic printing device are shown in FIG. 1. Therein, an image receptor, e.g., an organic photoreceptor, is provided on a drum 10 and driven by a motor (not shown) to rotate in the direction of the arrow. The drum 10 rotates the image receptor past a charging device 12, e.g., a corona device or other charging apparatus, which charges the image receptor on drum 10 to a predetermined voltage level. An image to be reproduced is generated on the charged image receptor using a suitable imaging device 14, e.g., a laser, which operates to discharge a portion of the charge which was placed on the image receptor by charging device 12. This results in the image receptor of drum 10 having an image portion and a background portion which are defined electrically by areas of different electric potential.
The latent image on the image receptor can be developed in multiple ways. Typical laser printers use toner powders as the developer. For devices with a liquid toning system, the developer is commonly referred to as ink or liquid toner. As used herein, the term “developer” includes toner powders, inks, liquid toners and the like. For liquid developers the image is usually formed by electrophoresis of charged ink particles onto the discharged regions of the photoreceptor. In an alternative liquid toning embodiment, an elastic developer roller 16 may be first coated with liquid developer particles by electrophoresis. Then the roller is urged against the imaged photoreceptor on drum 10 at the nip between drum 10 and development roller 16, the region proximate this nip also referred to herein as the “development zone”. Once the developer is provided to the image receptor on drum 10, the electrostatic latent image is developed and ready for transfer, via transfer roller 20 to a substrate 22, e.g., paper, carried by roller 24. Alternatively, the developed image can go directly from the image receptor to the substrate.
One advantage associated with using the development roller 16 to transfer developer to the image receptor on drum 10, rather than applying the developer directly to the drum 10 downstream of the imaging device 14, is that the operating speed of the system can be increased while still maintaining a sufficiently thick developer layer. However, use of the development roller has also resulted in certain challenges, including ghost images. An example is shown in FIG. 2 for a print made using a system similar to that of FIG. 1. Therein, a page 26 was printed entirely in cyan (which is shown in the Figure as grey) except for a rectangular patch 28 in which the cyan was omitted. There are two ghosting related problems with the printed page 26. First, the cyan in the lower portion of page 26 was printed with a lower density (lighter color) than the cyan in the upper portion of the page, despite the fact that the intended color density for the entire page was the same. Second, a ghosted rectangular patch 29 appeared symmetrically in the lower portion of the page 26 as a rectangular patch of cyan having a density which is higher than the surrounding region (the same density as in the upper portion of page 26). The upper portion of the page 26 has a length which corresponds to the circumference of the developer roller 16, with the developer roller 16 and imaged photoreceptor drum 10 moving at substantially the same surface speed. These circumstances indicate that the ghosting problems are associated with the development roller 16. More specifically, the ghosting problems illustrated in FIG. 2 may be associated with insufficient developer roller conductivity.
The development roller 16 is usually made of an elastomeric polymer, such as polyurethane, doped with a conductive agent. These non-insulating elastomers are only moderately conductive, having a resistivity of around 106 ohm-cm or higher. While metals are much more conductive, they are typically not appropriate materials for the development roller 16 since good contact with the inelastic photoreceptor on drum 10 can only be ensured with an elastic roller. One solution to this problem is to increase the concentration of the conductive particles in the developer roller 16 to increase its conductivity. However, this solution may have drawbacks because higher concentrations may negatively impact the mechanical and chemical properties of the development roller. Accordingly, it would be desirable to provide systems and methods for printing which avoid the afore-described problems and drawbacks.